Coating substrates such as vehicle bodies are coated during a series of steps constituting a coating line while the vehicle bodies are being conveyed with hangers or carriages. The coating line involves at least a spraying step for spraying an intermediate coat or a top coat and a drying step for drying the coat sprayed on the vehicle body. The drying step may be broken down into a setting step and a baking step when a thermosetting paint or a two-part setting-type paint is employed as a coating paint. The setting step is designed to volatilize a solvent in a range of relatively low temperatures such as room temperatures to a sufficient degree, and the baking step is to bake the coat at elevated temperatures. In instances where a powder coating is employed as a coating paint, the drying step involves the baking step only because no such powder coating contains any volatile solvent.
The paint on the coating substrate is generally sprayed with a spray gun. The spraying is also effected from a transverse direction on a surface of the coating substrate extending in the vertical direction, hereinafter will bereferred to as a vertical surface. The spraying of the paint in the transverse direction allows a coating to be formed in a predetermined film thickness with accuracy.
A degree of evenness on a coated surface is determined as one of standards evaluating the quality of the coated surface. The degree of evenness gets higher as irregularities in the coated surface gets smaller, leading to a higher quality. It is known that a film thickness of a coat sprayed on a coating substrate gets thicker as a higher degree of evenness is achieved.
However, when a paint is sprayed on a coating surface, the paint is caused to sag on the coated surface and such sages impairs a quality of the coated surface. The sags may be caused to occur as the paint sprayed flows downwardly or droops by the gravity so that a film thickness of the paint sprayed gets thicker as the sags are more likely to occur. As the sags occur by an influence of the gravity, they may be likely to occur on a coated surface extending in the downward or upward direction such as the vertical surface. On a surface of a coating substrate extending in the horizontal direction, or a transverse surface, causing no big problems with sagging may be formed a thicker film than a film coated on the vertical surface. If a film thickness of a coat formed on the transverse surface is as thick as that of a coat formed on the vertical surface, the former can provide a degree of evenness higher than the latter because the paint coated on the transverse surface is caused to flow to such an extent that it causes no sags.
Heretofore, attempts have been made to prevent a coated paint from sagging and at the same time to provide a degree of evenness as high as possible on the coated surface by using a paint with a possibly lower degree of flowability. A sagging threshold value or a limit on a film thickness of a paint coated causing no sags is known to be as thick as 40 .mu.m for a thermosetting paint although the sagging threshold value varies with kinds of paints. Accordingly, in instances where a thermosetting paint is employed as a coating paint, a film thickness to be coated on the vehicle body in the spraying step is determined such that no sags are caused to occur at the early stages of the setting step and the baking step, particularly at the early stage of the baking step because the sags are likely to occur at these stages. Thus, in order to form a coated surface with a higher degree of evenness, it is necessary in conventional spraying procedures to plurally effect the spraying or repeat a series of steps from the spraying step to the baking step. From the different point of view, a predetermined film thickness of a paint coated immediately after the spraying can be controlled with accuracy in the spraying technique so that the film thickness is rendered as thicker as possible within a range that causes no sags.
In instances where a two-part setting-type paint is used, on the one hand, sags are likely to occur in the setting step and a sagging threshold value for a two-part setting-type paint is as thick as approximately 40 .mu.m. In instances where a powder coating is used, on the other, the paint is most likely to sag in the baking step and a sagging threshold value for it is as thick as approximately 80 .mu.m. As thermosetting paints and two-part setting paints flowable at room temperature is extremely high in flowability and low in viscosity, sags are likely to occur immediately after they were sprayed. The same thing can be said when a paint is sprayed too much.